During the period of October 1, 1990 - March 31, 1991, the particular phase of the project that concerns Deformability of Red Blood Cells using Electron Paramagnetic Resonance (EPR) was reviewed. This phase of the general project. Structure and Interaction of Biomolecules, started in 1980 with a novel idea of using spin labeling technique for probing the slow dynamics of macroscopic (e.g. cellular) state, while the spin labeling technique had been used exclusively to study the molecular (microscopic) motional state and its change. Our idea was based on the change of EPR spectral shape derived by partially orienting the cells in shear flow, the ease of orientation being directly related to the ease of cell deformation by flow. First, a theoretical model with the cell deformability included as a parameter was constructed to explain and simulate the observed EPR spectra, enabling to estimate the degree of orientation and deformation index from spectral changes in flow. Decay in time of the flow-induced spectral change was also used to judge the cell deformability. The method was applied to study red blood cells in the states where the cell deformability was altered for various reasons: for example, the effects on the cell deformability of spin labeling itself, cross-linking by diamide, glutaraldehyde and other chemicals, effect of intracellular Ca2+, Cu2+ ions, Heinz body formation, hematocrit, effect of co-existing hardened cells. Red cell ghosts were also studied with a specific interest in finding among various modifications of ghost preparation method the one that gives the best deformation properties, and finding a possible relation between an altered state of cytoskeletal protein spectrin and flow properties of ghost. in most cases, the factor(s) were identified in the cell structure that cause the deformability change.